Shape-Induced Dispersion of Colloids in Anisotropic Fluids

We experimentally study the behavior of micrometer-sized prolate ellipsoidal particles dispersed in a nematic liquid crystal. The latter is an aqueous solution of rodlike micelles. When embedded into such a solvent, ellipsoids with small enough aspect ratios aggregate to form anisotropic structures oriented at an angle with respect to the local background director (as already observed for spheres). This is, however, no longer the case when the aspect ratio reaches a well-defined value: above that value, the ellipsoids remain well dispersed and apparently do no interact with each other, even over very long periods of time (several months). Therefore, there exists a transition from an aggregated to a nonaggregated state as a function of aspect ratio and for a given particle concentration. This behavior has not been predicted so far and we put forward simple calculations to rationalize our observations

Photonic Control of Surface Anchoring on Solid Colloids Dispersed in Liquid Crystals

International audienceThe anchoring of liquid-crystal (LC) mesogens to the surfaces of colloids is an important factor in determining intercolloidal interactions and the symmetry of the ensuing colloidal assembly in nematic colloids. The dynamic control of surface anchoring could therefore provide a handle to tune the colloidal organization and resulting properties in these systems. In this article, we report our results on the study of thermotropic nematic LC (E7) dispersions of silica and glass microcolloids bearing photosensitive surface azobenzene groups. By the photoinduced modulation of the colloidal-LC interfacial properties, due to the trans-cis isomerization of azobenzene units, we tune the anchoring on silica colloids from homeotropic (trans-azobenzene) to homogeneous planar (cis-azobenzene) reversibly. In tune with the change in surface anchoring, the interparticle interactions were also dictated by dipolar and quadrupolar symmetries for homeotropic and homogeneous planar anchoring, respectively. In our experiments, we find that, in addition to the isomerization state of the surface-bound azobenzene units, the nature of the colloid plays a crucial role in determining the anchoring state obtained on applying photostimuli. We also study the LC anchoring on colloids as a function of the azobenzene surface density and find that beyond a threshold value the anchoring properties remain invariant

Photonic Control of Surface Anchoring on Solid Colloids Dispersed in Liquid Crystals

The anchoring of liquid-crystal (LC) mesogens to the surfaces of colloids is an important factor in determining intercolloidal interactions and the symmetry of the ensuing colloidal assembly in nematic colloids. The dynamic control of surface anchoring could therefore provide a handle to tune the colloidal organization and resulting properties in these systems. In this article, we report our results on the study of thermotropic nematic LC (E7) dispersions of silica and glass microcolloids bearing photosensitive surface azobenzene groups. By the photoinduced modulation of the colloidal-LC interfacial properties, due to the trans–cis isomerization of azobenzene units, we tune the anchoring on silica colloids from homeotropic (<i>trans</i>-azobenzene) to homogeneous planar (<i>cis</i>-azobenzene) reversibly. In tune with the change in surface anchoring, the interparticle interactions were also dictated by dipolar and quadrupolar symmetries for homeotropic and homogeneous planar anchoring, respectively. In our experiments, we find that, in addition to the isomerization state of the surface-bound azobenzene units, the nature of the colloid plays a crucial role in determining the anchoring state obtained on applying photostimuli. We also study the LC anchoring on colloids as a function of the azobenzene surface density and find that beyond a threshold value the anchoring properties remain invariant

Protein-Induced Configuration Transitions of Polyelectrolyte-Modified Liquid Crystal Droplets

Liquid crystal (LC) droplets dispersed in aqueous solution have emerged as an optical probe for sensing the adsorption and interaction of biological species at the LC/aqueous interface. In this paper, we modify the surface of 4-n-pentyl-4′-cyanobiphenyl (5CB) LC droplets by the adsorption of positively charged poly(diallyldimethylammonium chloride) (PDADMAC) and poly(ethylenimine) (PEI) with different molecular weights at the 5CB/water interface. The PDADMAC and PEI-modified 5CB droplets show a radial director configuration in aqueous solution with salt concentrations above 150 mM. The adsorption of negatively charged bovine serum albumin (BSA) on the positively charged PDADMAC and PEI-modified 5CB droplets through electrostatic interaction can induce the radial-to-bipolar configuration transition of the 5CB inside the droplets. We find that the concentration of BSA required to induce the configuration transition increases linearly with the decrease of the molecular weight of PDAMAC and PEI. Our results highlight the capability of the director configuration of LC droplets as an optical probe for sensing the interaction between proteins and polyelectrolytes at the LC/aqueous interface. © 2014 American Chemical Society

Do thermal diffusion and Dufour coefficients satisfy Onsager’s reciprocity relation?

It is commonly admitted that in liquids the thermal diffusion and Dufour coefficients DT and DF satisfy Onsager's reciprocity. From their relation to the cross-coefficients of the phenomenological equations, we are led to the conclusion that this is not the case in general. As illustrative and physically relevant examples, we discuss micellar solutions and colloidal suspensions, where DT arises from chemical reactions or viscous effects but is not related to the Dufour coefficient DF . The situation is less clear for binary molecular mixtures; available experimental and simulation data do not settle the question whether DT and DF are reciprocal coefficients

Liquid Crystal Templates Combined with Photolithography Enable Synthesis of Chiral Twisted Polymeric Microparticles

Liquid crystals (LC), when combined with photolithography, enable synthesis of microparticles with 2D and 3D shapes and internal complexities. Films of nematic LCs are prepared using mixtures of reactive (RM257) and non-reactive mesogens with controlled alignment of LCs at the confining surfaces, photo-polymerized the RM257 using a photomask, and then extracted the unreacted mesogens to yield the polymeric microparticles. The extraction results in a controlled anisotropic shrinkage amount dependent on the RM257 content and the direction dependent on LC alignment. Control over the aspect ratio, size, and thickness of the microparticles are obtained with a coefficient of variance less than 2%. In addition, non-parallel LC anchoring at the two surfaces results in a controllable right- or left-handed twisting of microparticles. These methods may find substantial use in applications including drug delivery, emulsions, separations, and sensors, besides their potential in revealing new fundamental concepts in self-assembly and colloidal interactions